The invention concerns a hydraulic steering system for a vehicle with two steering motor arrangements, particularly an articulated vehicle with wheel steering, with a hydraulic steering unit having a measuring motor section, with a selection valve, which connects an outlet of the steering unit with either a first steering motor arrangement or a second steering motor arrangement and which is hydraulically operable via a pilot valve, and with a pump, which is connected with the steering unit and the pilot valve.
A conventional steering system is shown in U.S. Pat. No. 4,802,545. Such steering systems are used for tractors, building site vehicles or other working machines, for which a very small steering radius is desired. A vehicle of this kind has two frame parts, which are connected with each other via a converting kit. When, via the second steering motor arrangement, the two vehicle parts are brought to an angle position in relation to each other, the vehicle will drive in a curve. When, additionally, it is ensured that at least one wheel pair can be turned in relation to the vehicle part, on which it is arranged, the curve described by the vehicle will be even narrower. The switching between the two steering motors is made by means of a selection valve, which lets the steering unit act upon either the first steering motor arrangement and thus upon the steered wheels, or the second steering motor arrangement and thus upon the converting kit. This selection valve is hydraulically operated, the pressure required for the operation being controlled by a pilot valve.
Considering the tough working condition to which such working machines are exposed, failure situations cannot always be avoided with the required reliability. Thus, in certain cases, it may happen that for some reason the pump pressure fails. When, in such a situation, the vehicle is articulated, it can practically no longer be steered. The measuring motor section of the steering unit can be used as auxiliary pump, to operate one of the steering motor arrangements. As, however, due to the missing switching ability of the selection valve the other steering motor arrangement can no longer be operated, the vehicle on a whole can practically no longer be steered. This is particularly critical, when the vehicle can no longer be straightened from its articulated position, as then, under certain circumstances, a towing is also no longer possible.
Therefore, the principal object of this invention is to permit steering of the vehicle in the case of an emergency.
The steering system of this invention has an operating inlet of the pilot valve connected with an outlet of a change-over valve, whose first inlet is connected with the pump and whose second inlet is connected with the outlet of the steering unit.
This embodiment solves the task of permitting steering of the vehicle in case of an emergency with relatively little effort. It must merely be ensured that in the case of failure of the pump pressure, the operating inlet of the pilot valve can be supplied with the pressure from the steering unit. This is the purpose of the change-over valve. Basically, therefore, this change-over valve and an additional line are required to make the selection valve operable by means of the pilot valve. When the selection valve is operable, the driver can operate both steering motor arrangements and bring the vehicle into a state, in which it can be steered more easily. A preferred application for the invention is vehicles in which the first steering motor arrangement acts upon steered wheels and the second steering motor arrangement acts upon a converting kit.
The change-over valve is made as a two-way valve. The two-way valve passes the higher of the pressures on its two inlets on to its outlet. During normal operation, the higher pressure is the pump pressure, so that in the undisturbed case the pilot valve can always pass on the pump pressure to the selection valve. When, however, the pump pressure fails, the higher pressure is supplied by the steering unit, so that now the pressure of the auxiliary pump reaches the operating inlet of the pilot valve, and, when the pilot valve is operated, the pressure of the auxiliary pump can change over the selection valve. This solution has the advantage that the driver does not have to consider the change-over. When the pressure of the pump fails, but the steering unit is operated, the pressure for changing over the selection valve is automatically available. When the fault is remedied and the pressure of the pump is available again, the return to the pump pressure takes place automatically.
The outlet of the steering unit has two directional lines, between which a second two-way valve is arranged, whose outlet is connected with the second inlet of the change-over valve. The pressure for changing over the selection valve is thus available, independently of the direction, in which the vehicle is steered. Also here, changing over to the directional line with the higher pressure takes place automatically, so that an intervention by the driver is not required.
The measuring motor section can be switched between two different displacements by means of a pressure controlled second change-over valve, the second change-over valve setting the smallest displacement in the pressureless state. In this connection the fact is taken into consideration that usually the steering motor arrangement, which acts upon the steered wheels, has a smaller fluid requirement than the second steering motor arrangement, which acts upon the converting kit. When the displacement of the steering unit is changed, it may be ensured that the driver of the vehicle feels no large differences, when steering the vehicle by means of the wheel steering or by means of the converting kit. When, however, in the case of a failure of the pump pressure, the measuring motor section is used as auxiliary pump, a large displacement means an equally large effort for the driver. When it is ensured that a failure of the pump pressure will automatically cause the setting of the smaller displacement, the driver will need less force for the emergency steering. The selection of the smaller displacement takes place automatically, so that the driver no longer has to take care of the change-over. In unusual situations, as for example emergencies, it can usually not be assumed that the driver considers all opportunities.
A sensor is arranged on the converting kit, which sensor is connected with a control device, which separates the operating inlet of the pilot valve from its outlet when the neutral position of the converting kit is reached. Here, it is assumed that in the case of an emergency the steering should mainly be made by means of the steered wheels. The second steering motor arrangement is thus only operated, until the vehicle is straightened again. This position is recognised by the sensor and reported to the control device. The control device may for example contain some electronics activating the pilot valve electrically. During failure of the pump pressure, the control device can then overrule a signal from the driver, who wants to operate the converting kit.
In the pressureless state the selection valve connects the first steering motor arrangement with the steering unit. In the case of an emergency, the wheel steering is the preferred mode of steering. This is then automatically selected. An operation of the selection valve is only required in the cases, in which the vehicle is not straight.